The multiple-input multiple-output (Multiple-Input Multiple-Out-put, MIMO) technology is a breakthrough in antenna technologies in the wireless communications field. With this technology, the capacity and reliability of a system can be improved by several times without increasing usage of time and frequency resources.
A split-type microwave device mainly includes the following parts: an antenna, an ODU (Outdoor Unit, outdoor unit), an intermediate-frequency cable, and an IDU (Indoor Unit, indoor unit). The antenna is configured to receive a carrier signal sent by the ODU and increase signal gain; the ODU is configured to perform radio frequency processing to implement signal conversion between a radio frequency and an intermediate frequency, and superpose an intermediate-frequency signal with a radio-frequency signal to form a carrier signal and send it to the antenna; the intermediate-frequency cable is configured to transmit an intermediate-frequency service signal, and a communication signal between the IDU and the ODU, and supply power to the ODU; and the IDU is configured to implement service access, service scheduling, multiplexing, modulation and demodulation and intermediate-frequency signal processing.
If the split-type device supports the multiple-input multiple-output technology, a plurality of ODUs and one IDU are required. The IDU connects to the plurality of ODUs respectively, and each ODU has an independent radio frequency-reference crystal oscillator (a local oscillator for short) and a same first frequency multiplication factor M, where M is the ratio of a transmit frequency design value of a remote radio frequency unit corresponding to the ODU to a frequency of the radio frequency-reference crystal oscillator. As a result, not only there is a deviation between a transmit carrier frequency offset and a receive carrier frequency offset of each channel, but also there is a deviation between carrier frequency offsets of different channels (carrier frequency offset for short). The carrier frequency offsets of different channels superpose at a receiver, which greatly increases the complexity of a joint frequency offset correction algorithm at the receiver, and a large carrier frequency offset may even lead to correction failure or degraded system performance.